Scientists at the Naval Research Laboratory have discovered a nanowire lattice that self-assembles from materials used in next-generation infrared detectors. The scientists found that a three-dimensional "crystal" of indium arsenide nanowires spontaneously forms when a superlattice of alternating indium arsenide (InAs) and gallium antimonide (GaSb) thin films is grown by molecular beam epitaxy.

The striking structure of the nanowire lattice, revealed with atomic-resolution using cross-sectional scanning tunneling microscopy (X-STM), is reported in the December 2, 2002 issue of Applied Physics Letters.

According to the research team, Drs. Brett Nosho and Brian Bennett of the Electronics Science and Technology Division, and Dr. Lloyd Whitman of the Chemistry Division, "we discovered the unusual nanostructure while attempting to optimize the fabrication of infrared detectors made from InAs/GaSb superlattices." The detectors, under development to help defend the nation against a ballistic missile attack, are created by depositing alternating films of InAs and GaSb (or InGaSb), with each thin film typically less then 15 atomic layers thick (<5 nanometers). "Usually," says Dr. Bennett, who led this portion of the study, "we try to make each film as flat and distinct as possible."

The scientists discovered that when the arsenic flux is much higher then the indium flux during the InAs film growth, something unexpected happens. As described by Drs. Nosho and Whitman, experts in X-STM, "Each metal-like InAs layer spontaneously forms a hill-and-valley structure, creating periodic nanowires about 10 nanometers high, 120 nanometers wide, and many micrometers long." "Moreover," continue the scientists, "the next layer of InAs nanowires align between the previous ones, not on top, as usually seen in these types of systems." A cross-section through the wires appears like a face-centered cubic lattice.

The InAs has a slightly smaller lattice constant then the GaSb substrate on which the superlattice was grown, and the nanowire lattice appears to arise from the resulting misfit stress combined with specific InAs and GaSb growth kinetic effects. "The very anisotropic nature of the lattice should give the material interesting optical and electrical properties," the scientists conclude. In collaboration with physicist Donna Stokes at the University of Houston, they are currently studying these properties.

X-STM is performed by cleaving a super-lattice sample in situ and imaging the
structure in cross-section along the edge of the sample.

The three-dimensional structure of the InAs nanowire lattice. (a) Atomic force
microscopy image of the growth surface. (b) Illustration of the structure.
(c) and (d) X-STM images of the two orthogonal cleavage surfaces. The InAs
are the darker layers.

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